This invention relates to filters and filtration generally, and more particularly to that type of filter known as an electrically stimulated filter (ESF), that is, the type of filter wherein the particulate-filtration efficiency of a mechanical filter is enhanced electrically. Electrically stimulated filters are attractive because they result in lower flow restriction and/or higher flow rate per filter area, with generally higher contaminant-holding capacity in comparison with purely mechanical filters of similar efficiency.
Many of the conventional electrically stimulated filters have in common the aspect of utilizing separate ionizing (charging) and collecting electrical fields. Examples of such conventional devices are found from U.S. Pat. Nos. 3,798,879 and 4,357,150 as well as from Canadian Pat. No. 821,900. Another device of this type is disclosed in co-pending U.S. patent application Ser. No. 48,452 of Jaisinghani et al filed May 11, 1987, the disclosure of which is hereby incorporated by reference herein.
Conventional two-field (i.e., separate ionizing and collecting electrical field) devices typically require from 3-5 electrodes, with two of the electrodes being maintained at high voltage. Additionally, some devices utilize only a single electrical field, which is either an ionizing field for charging particles, or an electrical field for polarizing or charging the filter material. U.S. Pat. No. 4,357,151 discloses an example of a filtration device relying solely on charging particles and collecting the charged particles on a non-electrified filter (collector). U.S. Pat. No. 2,297,601 discloses an example of a device which relies primarily on polarizing the filter material and not charging the incoming particles, although it is also disclosed that the device can be used with a separate ionizing field.
The present invention is concerned with utilizing a single high voltage electrical field for enhancing the filtering efficiency of a non-conductive filter medium. The single high voltage electrical field is used both to charge the incoming particles and to charge and polarize the filter medium. Thus, in comparison to a conventional non-electrified mechanical filter having the same filtration efficiency, the electrically stimulated filter of the present invention provides a significant advantage in flow rate, pressure drop and contaminant holding capacity or life, for a given amount of filter material.
Furthermore by the single field electrically stimulated filter of the present invention, these advantages may be obtained more economically, due to the utilization of only one electrical field, and to the requirement for only one high voltage electrode, as compared to the conventional two-field electrically stimulated filters. Furthermore, the single field ionizing electrically stimulated filter according to the present invention provides enhanced filtering efficiency at lower power consumption, that is, at approximately the same power consumption as in the conventional two-field ESFs, higher efficiency enhancement is possible in accordance with the present invention.
The method and apparatus of the present invention provide ionizing electrically stimulated filtration and thus may be referred to as providing an ionizing electrically stimulated filter (IESF). The principal of operation of the IESF according to the present invention is depicted schematically in FIG. 1. A non-conductive porous/fibrous filter "F" is placed within an ionizing field. The ionizing field may typically be achieved by the use of thin ionizer wires "W" spaced apart and maintained at a high potential. The non-conductive filter media "F" is placed between the high voltage wires "W" and a flat perforated electrode "E" of opposite polarity, typically at ground potential, as shown in FIG. 1.
The non-conductive filter material "F", being a dielectric, tends to suppress the field ionization in comparison to the field ionization that would occur without the presence of the filter medium dielectric. However, some ionization still occurs due to the porous nature of the filter medium. This lower level of ionization, however, is still sufficient to adequately charge incoming comtaminant particles. Charged comtaminant particles and ions collect on the upstream side of the filter material "F", producing a differential charge and potential across the filter medium "F". Since the filter medium "F" is non-conductive, these induced charges dissipate very slowly.
Also, due to the presence of the non-conductive filter medium "F" within the ionizing electrical field, there is always a potential difference maintained across the filter medium "F" independent of the amount of captured charged particles on the upstream side. This potential difference depends on the potential applied to the ionizer wires, "W", and on the surface and volume resistivity of the filter medium "F". Thus, a highly non-conductive filter medium is to be preferred. Best results are obtained when the downstream side of the non-conductive filter medium "F" is in contact with the perforated ground electrode "E". Filter media contact or close proximity with the ionizing wires or ionizing electrodes "W" must be avoided at all costs, since such contact or proximity tends to almost completely suppress the ionization, and thus tends to reduce the device to operating solely as a simple polarized filter medium.
Another aspect of the IESF according to the present invention is that the dielectric constant of the filter media also affects the filter's performance. Higher dielectric values result in increased particle capture due to dielectrophoresis (i.e., the interaction of polarized contaminant particles and the polarized fibers of the filter medium). However, higher dielectric value materials also tend to have higher conductivity, and this tends to lower the potential difference which is obtained across the filter medium and thus lower the efficiency enhancement which is obtained from electrical stimulation. We have found that typically greater than 10.sup.12 ohm-cm value resistivity is preferred and that this can be achieved with filter media having a dielectric constant of typically between 2-5. Preferably, glass fibers of resistivities between 10.sup.13 -10.sup.16 ohm-cm are utilized, although other materials such as polypropylene and polyester and the like can be used.
The filter medium should also be able to withstand some level of corona discharge and not ignite in the electrical environment. The severity of this aspect of the electrical environment depends on the applied field strength and on the gap "g" between the ionizing wires "W" and the non-conductive filter medium "F". The smaller the gap "g" and the higher the applied field strength, the greater is the necessity that the filter medium "F" be resistant to the corona discharge. When the ionizing Wires "W" touch the filter "F" or are in too close proximity thereto, almost all common filter media fail at all practical applied field strengths (i.e , field strengths required for adequate ionization). Thus, it is very important to maintain a gap between the ionizing wires and the filter medium.
U.S. Pat. No. 2,973,050 discloses a gas cleaning filter that does utilize a single electrical field. However in this device it is required that the collecting medium be conductive. As noted above, if the collecting medium is conductive, then potential difference which can be obtained across the filter medium drops markedly, i.e., to zero for a metal conductor as described in this prior patent, and the efficiency enhancement of the device will also drop. The device according to this prior patent operates as an ionizing-only field device where the electrical enhancement is due to the interaction of charged particles with an uncharged non-polarized filter medium.
However, it is well known that the interaction between charged and polarized particles and charged/polarized filter media is significantly higher than the interaction of charged particles with an uncharged non-polarized filter medium, as has been described previously in C N. Davies, "Air Filtration", Academic Press, New York (1973). Further, it is well know that the interaction between charged and polarized particles and charged and polarized filter media is significantly higher than the interaction of uncharged, polarized particles and polarized filter media. In particular the interaction between charged and polarized particles and charge polarized filter medium has only been achieved previously by the two-field ESF devices, and may now advantageously be achieved more efficiently by the IESF method and apparatus according to the present invention.
U.S. Pat. No. 4,244,710 also discloses a filter unit utilizing only a single electrical field, but requires the utilization of a charcoal filter. Since charcoal is highly conductive (being neither a non-conductor nor a dielectric), this use of charcoal as a filter media closely corresponds to the use of the conductive filter media in the device disclosed in U.S. Pat. No. 2,973,054 as discussed above.
Another single field device is described from U.S. Pat. No. 3,763,633. In this prior device, it is required that the "ionizing" electrode make contact with or be in close proximity to the filter medium. More particularly, in this prior device it is required that the filter medium "dielectric foam" be sandwiched between the "ionizing" wires and the ground electrode. In particular in this prior device the dielectric foam filter media which sandwiches the high potential electrode screen is also compressed in contact against a conductive foil prefilter which in turn is in electrical contact with a front ground electrode. However, as discussed above, such filter medium-ionizing electrode contact greatly reduces the ionization and filter polarization and reduces the enhancement mechanism to that of a device relying solely on the interaction between uncharged particles and polarized filter medium, without charging the particle. Furthermore, the device of this prior disclosure is intended to be used with significantly thick fibrous mat filter media. However, in most high efficiency filtration applications HEPA glass filter media is used for the removal of submicron size particles. This HEPA filter media is provided in sheet form having thicknesses of typically less than 0.5mm.
HEPA media in sheet form is very dense compared to glass-fiber mats, and this greater density tends to suppress ionization drastically, especially when the ionizer is in close proximity to the filter medium. Thus, the device described in U.S. Pat. No. 3,763,633 wherein the ionizer is in close proximity to the filter medium offers no significant advantages over the purely mechanical filtration efficiency, as has been shown from the results of evaluations in several cases as set forth in Table 1.
With reference to Table 1, a HEPA glass filter medium was utilized in a flat sheet form and evaluated for the following cases
Case (a): No applied electrical fields, (mechanical efficiency only being evaluated);
Case (b): Ionizing wires placed in close proximity to the HEPA medium, with a ground electrode placed in loose contact with the filter (equivalent to the device of U.S. Pat. No. 3,763,633);
Case (c): No ionizer utilized, but with filter medium sandwiched between two perforated electrodes in close proximity/loose contact with the filter medium, with one electrode maintained at high potential and the other electrode grounded ("equivalent" to an ESF without ionizing precharger);
Case (d) Ionizer in close proximity with the filter medium, and a ground electrode spaced 0.75 inches distant from the filter medium (somewhat similar to the device of U.S. Pat. No. 3,763,633); and
Case (e) single field ionizing electrically stimulated filter (IESF) according to the present invention, having a ground electrode in loose contact with the filter medium, and an ionizer spaced approximately 0.75 inches away from the filter medium.
Each of the cases (a)-(e) was evaluated at a fixed flow velocity of 66.6 feet/minute and at various applied voltages. The results are shown in Table 1.
TABLE 1 ______________________________________ Voltage Current Efficiency Case KV mA at 0.3 m DOP ______________________________________ (a) N/A N/A 67% (b) 2.5 0 72% (b) 11 0.22 71.2% (c) 2.5 0 69% (c) 6.25 0 74% (d) 11 0.002 73% (e) 11 0.18 99.3% ______________________________________
From the results in Cases (b), (c) and (d), it may be seen that those arrangements according to the prior art do not offer any significant advantage over solely mechanical filtration when using an HEPA filter medium as in Case (a). From comparing Cases (b) and (c), it may be seen that the ionization is totally supressed by the proximity of the HEPA filter medium. The ionizer wires in Case (b) do not provide any enhancement over Case (c) utilizing the simple polarized media and non-ionizing perforated metal electrodes. Further, it is clear that even when the ground electrode is spaced apart from the media as in Case (d), the ionization is still supressed by the proximity of the filter medium to the ionizer wires, and thus little enhancement in 0.3 um DOP efficiency results. The significant enhancement achieved by the IESF of the present invention may be seen from comparing the 99.3% efficiency in Case (e) to the 71-73% efficiency in Cases (b) and (d).
Thus, we have found that the provision of a significant gap "g" between the ionizer and the filter medium is critical for enhanced efficiency. The single field ionizing electrically stimulated filter according to the present invention can conveniently use pleated or convoluted filter media. In this case, the ground electrode is placed in contact with or in close proximity to the downstream peaks of the filter medium while the ionizer wires are spaced away from the opposite peaks of the pleated media by the gap "g" as shown in FIG. 1. Such a configuration derives full benefit from the increased surface area presented to the flow by a pleated filter. Typically approximately 20,000-30,000 volts (KV) are applied when using a filter medium having 1.75-2" deep pleats with a total electrode separation of about 2.5-3". Such an arrangement results in an efficiency enhancement from about 50% (for a mechanical filter without any electrical field) to about 97-99% using 0.3 um DOP particles. Such an enhancement in efficiency is not possible with the conventional two-field ESF devices. For example, at practicle applied power levels, the enhancement of the device disclosed in copending U.S. patent application Ser. No. 48,452 utilizing two fields is 97-99% when using a media of mechanical (no electrical field) efficiency of 65-70%.
Thus, we have found that the single field ionizing electrically stimulated filter according to the present invention provides the advantages of significantly enhanced filtration efficiency over the prior single and two-field electrically stimulated filter devices, and provides these advantages at significant economies over the prior devices.